The present invention relates to a sensor, in particular a thermal sensor, and a method for manufacturing a largely self-supporting membrane in connection with the sensor.
Thermal sensors are used to measure radiation or flows, and operate on the thermoelectric, pyroelectric, or thermoresistive principle. Micromechanical infrared sensors are obtained principally by wet etching of silicon wafers in anisotropic etching media. Reference is made in this connection, for example, to A. Oliver and K. Wise, Sensors and Actuators, 73 (1999), pages 222 to 231.
Alternatively, it has already been proposed in German Published Patent Application No. 198 43 984 to perform a-chip-surface-economizing anisotropic dry etching of the rear side of the wafer, or to use a purely surface-micromechanical patterning process, for example with a technique based on porous silicon.
The purpose in all instances is to manufacture an at least largely self-supporting membrane with poor thermal conductivity, in addition to a heat sink, for example the wafer xe2x80x9cmainland,xe2x80x9d in order, by way of suitable thermal pairs, to generate a temperature gradient, brought about for example by thermal radiation, between a thermally well-insulated and thus hot contact on the membrane on the one hand, and a cold contact, anchored on the mainland or connected thereto, on the other hand, and to measure it. The thermocurrent occurring in this context is then an indication of the absorbed quantity of radiation.
Known thermal sensors and the necessary associated electronic analysis systems are moreover usually of hybrid construction, i.e. the actual sensor element is separate from the electronic analysis system and is joined to it, for example, via bonding wires; or the sensor element is located, for example, as a layer on a ceramic substrate that serves at the same time as support for the electronic analysis system. A hybrid configuration offers definite cost advantages in the case of individual sensors.
In the field of interior sensing of motor vehicles, security technology, and residential automation, there is an increasing need for sensor arrays with greater and greater resolution. The spacing between the individual sensor elements that form the array must therefore continuously be decreased. When known sensor elements are used, however, the result is that connection of the individual sensor elements to the associated analysis and compensation circuits, for example by wire bonding, becomes more and more complex and laborious in terms of production engineering.
A first approach to an improved manufacturing method for micromechanical structures and sensor elements, the so-called xe2x80x9cadditive lost formxe2x80x9d technique, has already been proposed in German Published Patent Application No. 44 18 163. In this, firstly a metallic layer and a patterned sacrificial polymerxe2x80x94equipped, for example by plasma etching, with openingsxe2x80x94are applied as the polymer form onto a silicon wafer having an active electronic circuit. A metal pattern is then grown on in the region of the openings that were created, and lastly the sacrificial polymer is removed so that the grown metal patterns remain behind.
The present invention for manufacturing a largely self-supporting membrane, in particular for manufacturing a vertically integrated thermal sensor array having that membrane, has the advantage over the existing art of making possible a considerable simplification of the electronic control of the individual sensor elements on the membrane that is created. In particular, wiring is not necessary, and high densities of thermopiles or sensor elements can be achieved on the membrane layer. This allows a sensor array according to the present invention to have high spatial resolutions with simple electronic control.
In addition, the largely self-supporting membrane that is produced makes possible very good and defined thermal insulation between the base element located therebeneath and the membrane or the sensor elements located thereon, which can be interrupted locally by the contact columns that are produced.
In addition, it is advantageously possible to utilize established process technologies, facilities, and materials for the individual process steps, which yields cost and quality advantages.
It is particularly advantageous, in the case of the configuration of a thermal sensor or thermal sensor array, if the largely self-supporting membrane layer that is produced is made of a material with poor thermal conductivity as compared to a metal, in particular silicon nitride. It is thereby possible, by way of the contact columns that are produced, which advantageously are made of a material with good thermal conductivity such as, for example, a metal, to generate heat sinks in controlled fashion in the region of the contact columns, so that a temperature gradient is created between the contact columns and the regions remote from the contact columns, and is maintained for a long period of time.
It is further advantageous if a thermopile arranged on the membrane layer has at least two thermocouples or thermoelectric elements connected in series, whose thermojunctions are alternatingly in contact directly with a thermal contact column and with the membrane layer. The overall result is to create in the thermopile a particularly large, easily measurable thermocurrent as a function of the temperatures of the individual thermojunctions. For example, thermal radiation locally incident on the thermopiles is thereby easily measurable in terms of its intensity, and can be analyzed in terms of lateral intensity differences over the membrane layer with a high lateral resolution of up to 5 xcexcm.